1. Field of the Invention
The present invention relates to a wireless terminal apparatus, a control method for a wireless base station and a control method for a wireless terminal apparatus; and to a technique effectively applicable to a wireless transmission and reception technique for use in a wireless telecommunications system performing a fast Fourier transform (FFT) in a signal process for transmission and reception, such as an Orthogonal Frequency Division Multiple Access (OFDMA), for example.
2. Description of the Related Art
The OFDMA transmission system allocates one or a plurality of sub-carriers to each user (i.e., a terminal receiver) for carrying out telecommunications between a base station and a plurality of users as noted in a patent document 1 for example. Let it be considered the case of generating an OFDMA signal by an N-point inverse fast Fourier transform (IFFT) at a base station and a terminal receiver which receives the signal for example. The terminal receiver applies a fast Fourier transform (FFT) to a received time signal for converting to a signal of a frequency range, selects a sub-carrier signal in a band allocated to the terminal receiver itself and carries out a subsequent sub-carrier demodulation process. Therefore, the terminal receiver applies an FFT to time signals in the lump including data addressed to a plurality of users.
In the case of an OFDMA signal of an N-point IFFT size being generated at a transmitter (i.e., the base station), the receiver also performs an N-point FFT. If the receiver performs an FFT of less than N points (e.g., N/2, N/4, and so on), a sub-carrier allocated to the receiver itself is overlapped with that of another user, disabling a normal reception.
However, in the case of a base station generating a signal of a large FFT size (e.g., N=2048 points) and transmitting it, a receiver also performing an FFT of the same size is faced with a technical problem of increasing an FFT operation volume and calculation time, and the resultant power consumption, at the terminal receiver.
The above described technique disclosed by the patent document 1 exists as a conventional technique for reducing a power consumption associated with an FFT. The technique according to the aforementioned patent document 1 rearranges sub-carriers of a transmission signal in an interval of an nth power of two (“2”) followed by applying an inverse Fourier transform when telecommunications traffic is light, thereby making it possible to reduce the IFFT size of the transmission signal and apply an FFT in the same number of points as an IFFT at the transmission side also at the time of receiving the aforementioned transmission signal. By so doing, the number of points of an FFT within a transmission and reception system is tried to be reduced.
The technique according to the patent document 1, however, needs to rearrange sub-carriers of the users in the interval of an nth power of two (“2”) (i.e., arrange them alternately) in order to reduce the number of FFT points at the transmitter side, and therefore the technique according to the patent document 1 cannot be utilized in the case of allocating, and transmitting, an adjacent sub-carrier to each user. Besides, if a base station transmits in N/2 points for example, becoming the same N/2 points at the receiving side, and therefore the receiver is unable to change it to an FFT size of N/4 points or N/8 points, i.e., being less than that of the transmission side.
Meanwhile, a patent document 2 has disclosed a technique, for use in a wireless reception apparatus, judging a propagation path condition in the entire range of a usage frequency band of a multi-carrier signal received from a base station, identifying a frequency band having a good propagation path condition from among a usage frequency band of OFDM and notifying the base station of it, thereby attempting to accomplish reductions of a telecommunication data volume for a frequency scheduling and of a power consumption of the wireless reception apparatus.
The technique, however, does not refer to a change of FFT sizes at the wireless reception apparatus.
[Patent document 1] Laid-Open Japanese Patent Application Publication No. 11-346203
[Patent document 2] Laid-Open Japanese Patent Application Publication No. 2005-244958